Rotary coring devices have been developed to obtain core samples from subsurface earth formations adjacent wellbores. One rotary coring device utilizes a hydraulic motor to rotate a rotary coring bit to obtain a core sample. A drawback with this rotary coring device is that when the device operates at relatively high temperatures (e.g., greater than 350° F.), the viscosity of the oil driving the hydraulic motor decreases. When the viscosity of the oil decreases, an output torque of the hydraulic motor is reduced below a desired torque level. Further, a rotational speed of a rotor of the hydraulic motor is reduced below a desired rotational speed.
U.S. Pat. No. 6,371,221 describes a rotary coring device that utilizes a first electric motor for rotating a rotary coring bit and a second motor for linearly moving the rotary coring bit. A drawback with this rotary coring device is that when the rotary coring device is disposed several thousand feet underground, supplying power to two electric motors is extremely difficult due to large power losses in conductors extending from an above-ground power source to the rotary coring device.
Accordingly, the inventors herein have recognized a need for a rotary coring device that reduces and/or eliminates the above-mentioned deficiencies.